Microwave phase shifters are used in many applications where it is required to electronically control the transmission of microwave energy either within a system (through a microwave switch) or in free space (in an electronically scanned antenna). These phase shifters typically use either semiconductor (diode) or ferrite technology. Both approaches have various advantages with the diodes generally being superior for lower frequency, low power applications and the ferrite version preferred where high power and/or high frequency conditions are experienced. Both broad classes of phase shifters are presently extensively used in microwave and millimeter systems.
In general, ferrite phase shifters are preferable where high power, low insertion loss (at frequencies as high as X-band), and wide frequency ranges are desired. When one considers phase shifters which are capable of switching in less than 50 to 100 microseconds, only two ferrite devices are widely used: the reciprocal latching phase shifter (dual-mode) and the nonreciprocal twin-slab phase shifter.
Generally, these devices tend to fill separate niches. The twin-slab phase shifter is extensively used in applications where the system requires extremely rapid reconfiguratlon since this unit is capable of changing its setting within 1 to 10 microseconds (that is, 10 to 100 times as fast as the dual-mode phase shifter). The twin-slab phase shifter is, however, inherently a nonreciprocal device and many systems require transmit and receive operations to occur either simultaneously or sequentially within a very short period of time. In these applications, the dual-mode phase shifter is often used since it provides very nearly the same phase shift for both transmit and receive operation without the need for switching (a state change).
In certain applications, such as in radar systems, a need exists for a reciprocal phase shifter (i.e., one which provides substantially the same phase shift for both transmit and receive operations) which also has the capability of switching (i.e., changing state) very rapidly.
In a pulsed radar system, a non-reciprocal phase shifter, in order to operate in transmit and receive modes, must be switched between transmit and receive states. In this regard, such a phase shifter is set in the transmit state to generate a radar pulse, but must be toggled to the receive state in time to receive the pulse returning from a target. If the pulses are transmitted at a high pulse repetition frequency, such a non-reciprocal phase shifter can't be switched at a fast enough rate to receive the returning pulses.
Although the dual mode phase shifter is a reciprocal phase shifter and does not need to be switched between the transmit mode and receive modes, such a phase shifter has significant shortcomings for many radar applications. In this regard, in a system using dual mode phase shifters, a long time is required to change the position to which the antenna is pointing (relative to a system using twin-slab phase shifters). In order to rapidly change the phase front across the antenna, the setting of the phase shifters in the phase array must be changed rapidly. Such dual mode phase shifters thus do not perform optimally in antenna systems where it is desired to move the antenna beam very rapidly. The excess time required to change the antenna position with dual mode phase shifters limits the functions which the antenna system can perform.
The phase shifter of the present invention uniquely provides for both reciprocal operation and fast switching speeds. Reciprocal operation in the transmit and receive modes is achieved by employing two latching, toroidal nonreciprocal phase shifters; one for transmitting and one for receiving. Thus, transmit and receive operations may occur simultaneously without the need for switching phase shifter states. The exemplary embodiment of the present invention utilizes input and output circulating devices which include a Faraday rotator and septum polarizer for appropriately routing signals through one or the other of the phase shifters depending upon the direction of input signal propogation.
The phase shifter of the exemplary embodiment of the present invention also achieves fast switching since the latching, toroidal, nonreciprocal phase shifters are transversely magnetized devices and are disposed entirely within a waveguide so that the generated magnetic field is confined entirely within the waveguide. The ferrite phase shifting elements do not intersect the waveguide walls and, thus, during a switching operation (e.g., when the toroid magnetization state is changed due to application of the latching magnetizing current pulse), the magnetic field is not switched through conductive waveguide walls. Accordingly, eddy currents are not induced during a switching operation thereby allowing for fast phase changes to be accomplished (which are not limited due to eddy current delays). This configuration allows for switching to occur within a time period in the range of 1 to 10 microseconds.
Thus, the present invention provides a phase shifter structure which has properties heretofor unattainable by any prior art ferrite phase shifter, thereby permitting use in applications where prior art ferrite phase shifters cannot perform adequately. At the same time, the present invention has the advantages of existing ferrite phase shifters (as compared to diode phase shifters, i.e., it provides low loss operation at frequencies in the upper microwave and lower millimeter usage and handles high peak and average RF power).